As LEDs have progressed over the past ten years and have become capable of handling more power than their early predecessor indicator LEDs, one area that becomes critical to the proper operation and longevity of the LED is thermal management. As stated in the document “Thermal Design Using Luxeon Power Light Sources” (Application Brief AB05) by Lumileds LLC, which is hereby incorporated by reference herein in its entirety (hereinafter “Thermal Design”), the manufacturer of the Luxeon High Brightness LED: “Proper thermal design is imperative to keep the LED emitter package below its rated temperature.”
It is well known and a published fact that high brightness and high power LEDs need to be connected to an external heat sink for operation over extended periods of time. As stated by Lumileds in document “Luxeon Reliability” (Application Brief AB25), which is hereby incorporated by reference in its entirety:                “While the reliability of Luxeon Power Light sources is very high, adherence to the device maximum ratings is required. The overall product reliability depends on the customer's drive conditions and adherence to recommended assembly practices. As with any other type of LED, extreme junction temperatures caused either by excessive power dissipation, an abnormally high thermal path, or improper assembly can cause thermal overstress failures.”        
As used herein, the term “HB LED” means LEDs of all types, light emitting polymers, and semiconductor dies that produce light in response to current that needs to be connected to a heat sink for optimal operation. Additional benefits of utilizing a heat sink include operation in higher ambient temperatures and the promotion of an extended life of the HB LED.
New methods designed to reduce thermal overstress failures of HB LEDs that are available include the utilization of aluminum substrates. Presently in the industry today, the use of Metal Core Printed Circuit Boards (MCPCB) or products based on this technology such as T-Clad™ by Bergquist Company offers a means of extracting the heat from High Brightness LEDs. Essentially, an MCPCB is a PCB (Printed Circuit Board) that utilizes an aluminum plate as a body as opposed to FR4, polyimide and other PCB and flexible circuit materials.
The process of installing an LED on an MCPCB is as follows. The LED must be glued to the MCPCB via a thermally conductive adhesive that is electrically neutral. The surface of the LED is glued typically to a copper pad on the dielectric layer of the MCPCB. Looking at the layers included in the MCPCB on the surface is the copper pad, below that is a dielectric layer, below the dielectric is the aluminum substrate. Once the LED is glued in place, the LED leads are soldered to the MCPCB. In some cases the LED is not glued in place, rather the LED's leads when soldered attach the LED to the board.
The use of MCPCBs in LED applications is very expensive. Besides the high price, MCPCBs are on a limited basis being offered by only several manufacturers. The uses of MCPCBs also do not promote the best cooling of the HB LED device. Since in most cases it is required to mount the aluminum substrate to an additional heat sink, a third junction is created (see page 4 of “Thermal Design”), which increases the thermal impedance of the assembly, thus in the long run, the life and performance of the HB LED.
It is also known that the base of most HB LEDs used for heat sinking is not electrically neutral. Therefore, consideration must be taken to electrically isolate this electrically conductive area. The MCPCB technology offers the solution of inserting a dielectric layer between the LED and the aluminum substrate. While this dielectric layer boasts decent thermal conductivity, it also plays a negative effect in the extraction of heat from the HB LED. Heat must transfer from the HB LED die, to the HB LED, to the thermally conductive adhesive holding the HB LED slug to the MCPCB assembly, through the copper pad that the HB LED is mounted to, through the dielectric layer, through the aluminum substrate, and finally to an external heat sink which will dissipate the heat into the ambient air. At each point, there is increased thermal resistance, thus the extraction of heat could be drastically improved.
Looking to the future as HB LEDs become more powerful and package size is not drastically increased, the extraction of heat from the HB LED will become more and more critical. As an example, present HB LEDs offer a thermal resistance of approximately 15 degrees Celsius per watt at the area where the die attach combines with die and material to contact with the die attach, as seen on page 4 of “Thermal Design”. While a one watt LED sees internally a minor rise in temperature 15° C.) a 5 watt HB LED experiences a 75° C. rise internally inside the part (at the junction as described above), therefore leaving very little head room for the remainder of the thermal design as the LEDs have a maximum junction temperature typically in the area of 120–130° C. In order to heat sink a device such as a 5 watt HB LED, a minimum amount of thermal junctions will be required in order to assure proper extraction of heat from the HB LED.